A honeycomb structure made of a ceramic has been used in a catalyst carrier for an internal combustion engine, a boiler, a chemical reaction device, a reformer for a fuel cell and the like in which a catalyst function is used, a trap filter such as diesel particulate filter (hereinafter referred to as “(DPF)”) of fine particles in an exhaust gas, particularly diesel fine particles and the like.
The DPF as one type of filter is incorporated in the exhaust system of a diesel engine to trap and remove particulates included in the exhaust gas from the diesel engine or the like. Examples of a material forming the trap filter typified by this DPF include a ceramic, a metal and an alloy. As a typical example of the filter made of the ceramic, a honeycomb filter made of cordierite, silicon carbide, silicon nitride or aluminum titanate is known. In particular, a honeycomb structure filter including porous honeycomb segments formed of a sintered body of silicon carbide or silicon nitride has an advantage that thermal resistance, strength and thermal conductivity are high, whereas there has been a problem that the filter has low thermal shock. In consequence, as the filter is large, crack is easily generated in the filter. Therefore, in recent years, as means for avoiding damage due to the crack, a technology has been suggested in which a plurality of porous honeycomb segments are integrated to manufacture one large ceramic structure (e.g., Patent Document 1).
This honeycomb structure has a constitution in which a plurality of porous honeycomb segments made of silicon carbide or the like are bonded to one another with a bonding material, processed into a predetermined shape such as a circular sectional shape, and then coated with a coating material. Each of the honeycomb segments has a large number of through holes which are separated by porous partition walls and which extend through the structure in an axial direction. The ends of the adjacent through holes are alternately plugged. The end of one through hole on the one side is opened, whereas the end thereof on the other side is plugged. The end of the other through hole adjacent to the one through hole on the other side is plugged, but the end of the other through hole on the one side is opened. According to such a constitution, when the exhaust gas flows into the through hole from the opened end, the exhaust gas passes through the porous partition wall and is discharged from the other through hole. When the gas passes through the partition wall, particulates in the exhaust gas are trapped by the partition wall, so that the exhaust gas can be purified.
In this honeycomb structure, deposited soot is burnt and gasified, whereby the filter is regenerated, and the particulates in the exhaust gas can be trapped again.
However, when the ceramic structure is used as the filter, there has been a problem that the fine particles trapped in the filter are not completely burnt, and easily partially remain unburnt. Therefore, the conventional honeycomb structure has a poor efficiency in treating the exhaust gas.
It is considered that a cause for the occurrence of such a problem lies in that thermal conduction between the filters is disturbed to make a temperature difference in the ceramic structure, and a ceramic filter assembly is suggested in which the thermal conduction between the filters is not easily disturbed to provide equally an excellent heat distribution.
For example, it is suggested that the thermal conductivity of the bonding material is defined to decrease a maximum temperature difference between an inner segment and an outer segment, but it is difficult to decrease a temperature difference between the center of the outer segment and the vicinity of an outer periphery coating material (e.g., Patent Document 2). Therefore, there remains unsolved a problem that the temperature in the vicinity of the coating material of the outer segment does not easily rise and that the soot of the corresponding portion is not easily completely burnt.
In particular, there remains unsolved a problem that the temperature in the vicinity of the outer periphery coating material of the outer segment does not easily rise in the honeycomb segment having a high porosity (a low thermal conductivity) as compared with the honeycomb segment having a high thermal conductivity.
On the other hand, it is suggested that the ratio of the thermal conductivity of the coating material with respect to the thermal conductivity of the bonding material is defined as 0.1 to 0.8 to decrease the maximum temperature difference between the inner segment and the outer segment and to improve the insulation property of the honeycomb structure. However, the porosity of the coating material is increased to decrease the thermal conductivity of the coating material, so that a problem that the strength of the coating material lowers and that the coating material is cracked remains unsolved (e.g., Patent Document 3).
Patent Document 1: JP-A-2000-7455
Patent Document 2: JP-A-2001-162119
Patent Document 3: JP-A-2002-273137